Farid Chouery, a shoring and structural engineer, has a proposal to build a new Alaskan Way Viaduct without tearing down the existing one and without completely stopping traffic.

First new anchors would be built. Then high-strength steel columns would be raised and encased in high-strength concrete. These would become the viaduct's new support columns. Working from below, crews would drill horizontally through the existing decks one at a time, and insert new high-strength steel to reinforce new high-strength concrete girders. Temporary shoring would be constructed to support the decks during this time. At least one deck would always be open to traffic during construction.

The end result would be a new viaduct. Chouery, a Ballard native, estimates the project's cost would be around that of a rebuild, but would reduce the negative economic effect of demolition and road closure.

The plan is based on Chouery's experience on projects including retrofitting the parking garage at SeaTac Airport in the 1990s, improving Providence Hospital's foundations in the late 1980s, and designing the shoring for Harbor Steps. The Ballard native has 30 years of experience in the Seattle area.

The tunnel is simply too costly an option in Chouery's estimation.

"I don't believe the tunnel is right; the amount of money is absurd," Chouery said. A Seattle Times poll in October found a slim majority of Seattle voters favored rebuilding the viaduct rather than the tunnel option. Project cost was a determining factor in people's choice. A March poll produced similar results.

Those numbers could be very important. Governor Christine Gregoire has kicked the viaduct question back to Seattle voters. If the city's citizens opted for the tunnel option, Seattle would have to come up with any extra costs, which are expected to be at least $1.8 billion.

Last week Seattle City Council passed legislation recently declaring a rebuild to be at odds with the city's comprehensive plan. The council had intended to put the viaduct question to a vote last year, but changed its mind, and endorsed a tunnel. Councilmember David Della and Council President Nick Licata opposed the move.

Licata expects a tunnel's costs would far exceed the estimates.

"Most projects of this size and complexity run over," he wrote in a news release last March.

The city has threatened to raise the cost of a rebuild by over $2 billion by slowing permits, filing lawsuits and other means. Since Chouery's proposal would technically be an upgrade to an existing structure, it would require fewer permits, and might avoid some of this fight.

The soil composition of the waterfront could affect the cost of a tunnel as well, according to retired geotechnical engineer Roger Lowe.

The tunnel is, effectively, a trench, Lowe said. It is not being bored through uniform material. Rather a trench is being dug in loose soil with varying properties and being capped with a lid. It is often referred to as a "cut-and-cover tunnel", but it is not a true tunnel because there is nothing above it.

Trenches can be much more difficult to maintain than tunnels.

The water level can create problems for such a structure, according to Lowe. The loose soil along the waterfront is saturated with water. Water below the tunnel will want to rise to reach the common water level.

"This structure is like a boat, and if you allow the water level to rise, the boat's going to float out of the ground," Lowe said.

The soil composition in the area could be another potential problem. The geotechnical firm, Shannon and Wilson recently finished preliminary soil sampling, but does not have any results yet.

Even the best sampling methods are estimates of an area's composition, and the soil along the waterfront consists of a mix of fill, debris, rubbish, ballast, sawdust and more.

"Nobody really knows what was put into that space," Lowe said. "There are a great deal of unknowns" which could drive up the cost of a tunnel.

The varied and loose soil could cause a tunnel to fail during a severe earthquake.

"Because soil composition varies greatly along the route, the seismic responses will also vary greatly," Lowe said.

The tunnel would have water on one side and soil on the other. This could cause the tunnel to bend along its longitudinal axis, potentially pushing it past the breaking point.

"Even if we attempt to design permanent shoring on both sides of the tunnel, the tunnel will experience a snake-type, large deflection in an earthquake," Chouery wrote in e-mail to the Washington State Department of Transportation. "As we have seen in past earth slides, shoring behaves like spaghetti in these types of forces,, and the only catalyst to solve the problem are the anchors. The tunnel will behave like a vibrating...beam and major cracks will follow and possibly a catastrophe..."

Lowe believes the bending would be more erratic than a "snake-type deflection".

Properly anchoring the tunnel would raise the cost "to unreachable territories", according to Chouery. The Viaduct, however, can be adequately anchored to withstand an earthquake.

Dan Catchpole can be reached via bnteditor@robinsonnews.com

Viaduct Rebuild

Dan Catchpole